How to bleed your radiators: a quick quide

What does 'bleeding a radiator' mean?

Bleeding your radiators removes air pockets that get in the way of hot water circulating around your central heating system. These air pockets build up over time and prevent heat from reaching certain places. This happens because water is about 900 times denser than air and when the air is under even a small amount of pressure, the air will always float to the top. That means that once air bubbles get into the system (for example when you repressurise your boiler), if they reach a high point with no way out (like the top of a radiator), they will stay there and create cold patches at the top. That’s why it’s important to regularly check your radiators and bleed them as needed.

If, however, bleeding your radiators doesn’t fix the problem, you may need to call a Gas Safe engineer to drain and flush your radiators. Flushing is the process of removing magnetite that has accumulated in your heating system over time. Magnetite is the dissolved metal that appears as a black sludge in your radiators. We advise leaving this task to a specialist.

How do I know if my radiators need bleeding?

The most common symptom of air trapped in your radiators is that radiators are cold at the top but warm at the bottom. Sometimes air can also be trapped in the pipe, which can make the whole radiator cold to the touch. Another sign of this is rattling, banging, boiling noises you hear soon after turning the heating on or when radiators are in operation. All of these could mean it's time to bleed your radiators.

Step-by-step process of bleeding your radiators

1. Turn off your boiler and wait until the radiators feel cool to the touch (this usually takes up to half an hour). Always make sure your heating is turned off before attempting to bleed your radiators.


2. Find the bleed valve. It is usually situated at the top of the radiator and can be at either end. It's round and has a square section in the middle with two notches. This is where you put the bleed key.


3. Put a towel on the floor and place a mug below the bleed valve, as some water is likely to come out after all trapped air escapes. If you have wallpaper on the wall behind the radiator, you might want to protect that as well by placing a piece of cardboard or plastic behind the radiator.


4. Insert the bleed key and turn it anti-clockwise until air starts to escape (you will hear a hissing noise). Keep the valve open as the radiators fill up with water and turn the key clockwise as soon as water starts to escape to retighten the valve.


5. Repeat on other radiators that need attention. If you're bleeding multiple radiators, start with the one that's the furthest from your boiler. If you live in a two-story or multi-story home, start with the radiators on the first floor and work your way up.


6. Turn your heating back on and check if your radiators are now heating up correctly.


7. Check your boiler's pressure. It's normal that your heating system will lose some pressure as a result of bleeding your radiators. If the pressure gauge reads less than 1 mark bar, the pressure in the system is too low and you will need to top it up. Click below for more information on how to repressurise your boiler.

Repressurise your boiler

How often should you bleed your radiators?

It’s usually enough to bleed your radiators once a year so you have peace of mind that your heating system is working efficiently. The ideal time to do this is around October, just before cold weather is upon us, so you can ensure your system is working well before the temperature drops and the heating is on a lot more.

If you're having to bleed your radiators more often than once a year, there might be an underlying issue like a leak or faulty component somewhere in the system and you should call your installer or a plumber to inspect it for you.